The present invention generally relates to spacecraft control, and more specifically, to methods and systems for providing attitude and antenna steering control for spacecraft.
In geosynchronous orbit, the attitude of a spacecraft is typically controlled so that the yaw/roll plane remains parallel to the earth equatorial plane and the pitch axis remains aligned with the earth pole axis. For this attitude, the payload antennas do not require active steering to remain properly pointed at their ground targets. The foregoing standard attitude approach, however, may have a number of disadvantages. For example, the maximum solar array pointing error may result in a power loss up to 8.3%. Also, the transponder panel incident solar flux reduces the heat rejection capability by as much as 25%. These losses are typically compensated with commensurate increases in solar array and transponder panel area, which increase the spacecraft cost and mass.
One existing and commonly used strategy, known as the Sun-Nadir Pointing (SNP) strategy, can be used to address the limitations of the foregoing standard approach. Using SNP attitude steering, a time-varying yaw profile is implemented where the spacecraft may rotate almost ±180 degrees over an orbit. The yaw rotation keeps the sun vector oriented in the spacecraft yaw/roll plane; as a result, there is no sun exposure to the transponder panels. The solar arrays are controlled in such a way that they point directly at the sun at all times, thereby minimizing any cosine loss. SNP attitude steering also has its drawbacks. One such drawback is that it requires high spacecraft yaw rates when the angle between the sun vector and the orbit plane (denoted as the beta angle) becomes small. For example, when the beta angle is about ten (10) degrees, the maximum yaw rate is 0.024 deg/sec, and when the beta angle approaches zero (0) degrees, a near instantaneous one hundred and eighty (180) degree yaw flip is required at orbit noon and midnight. The high yaw rates result in the need for large reaction wheels with a corresponding mass penalty. Also, yaw pointing accuracy may be reduced due to the impact of gyro scale factor uncertainty. Because of these issues, standard SNP attitude steering is typically abandoned at low beta angles in favor of the standard attitude approach. However, even with a beta angle as low as ten (10) degrees, the standard attitude approach may still result in significant thermal radiator performance degradation (as much as 14%) and solar array power reduction (as much as 1.5%).
Hence, it would be desirable to provide methods and systems that are able to more efficiently effect attitude and antenna steering control, thereby increasing available solar array power and improving thermal radiator performance during attitude steering.